In the association of serine proteinases with their cognate substrates and inhibitors, an important interaction is the fitting of the P1 side chain of substrate or inhibitor into a preformed cavity of the enzyme called the S1 pocket. In turkey ovomucoid third domain, the P1 residue is Leu18. To study the effects of changes in the P1 residue, Leu18X variants were made. Here, His18 OMTKY3 is used. The goal of this project is to determine Ka as a function of pH for 18His OMTKY3 in free inhibitor and in complex with SGPB using NMR. The different Ka of 18His OMTKY3 in free inhibitor and in complex with SGPB will be interpreted in view of the charge effect at the P1 residue.